antibody structure and the generation of b-cell diversity
TRANSCRIPT
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ANTIBODY STRUCTURE AND THE GENERATION OF B-CELL
DIVERSITY
ANTIBODY STRUCTURE AND THE GENERATION OF B-CELL
DIVERSITY
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WHAT ARE ANTIBODIES?
* Antigen specific proteins produced by plasma cells
* Belong to immunoglobulin superfamily
* Located in blood and extravascular tissues, secretions and excretions
* Bind pathogenic microorganism and their toxins in extracellular compartments
* Secreted form of immunoglobulins
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WHAT ARE IMMUNOGLOBULINS?
* Antigen specific proteins produced by B lymphocytes
* Belong to immunoglobulin superfamily
* Bound to surface of B lymphocytes* Function as binding (receptor) sites for specific antigens
* Antigen receptor sites on mature B lymphocytes* IgM* IgD
* Membrane-bound form of immunoglobulins
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WHAT IS THE IMMUNOGLOBULIN SUPERFAMILY
* Proteins with structural feature first defined in immunoglobulins
* Characteristic structural feature* Sequence of Domains providing stable conformation
* Domain* Polypeptide (100 to 110 amino acids) chain folded into sandwich
(2 slices of bread) held together by disulfide bond
* IG superfamily members* Antibodies, B cell receptors, T cell receptors, MHC molecules and
others
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STRUCTURE OF ANTIBODIES
* Antibodies are glycoproteins composed of* Polypeptide chains and carbohydrate
* Monomeric structure* Polypeptide chains
* 2 identical heavy chains
* 2 identical light chains
* Polypeptide chains joined by disulfide bonds
* Carbohydrate
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STRUCTURE OF ANTIBODIES
* Polypeptide chains have variable and constant regions* Variable
* N (amino)-terminal of polypeptide chain
* Antigen binding site
* Constant * C (carboxyl)-terminal of polypeptide chain
* Binding sites for cell surface receptors and complement
* Structure represented by the letter “Y”
* Y shaped molecule cleaved by protease papain* Fragment antigen binding (Fab)
* Fragment crystallizable (Fc)
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CLASSES (ISOTYPES) OF IMMUNOGLOBULINS
* Classes based on constant region of heavy chains* Immunoglobulin A (IgA)
* Immunoglobulin D (IgD)
* Immunoglobulin E (IgE)
* Immunoglobulin G (IgG)
* Immunoglobulin M (IgM)
* Differentiation of heavy chains* Length of C region, location of disulfide bonds, hinge region,
distribution of carbohydrate
* Classes have different effector functions
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CLASSES (ISOTYPES) OF IMMUNOGLOBULINS
* Additional classification based on light chains* Kappa
* Lambda
* Each IG has either kappa or lambda, not both* IgG kappa
* IgG lambda
* No functional differences between light chains
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IT’S GREEK TO ME
Heavy chains, light chains and other molecules of the immune system identified using letters of the Greek alphabet
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THREE DIMENSIONAL STRUCTURE OF ANTIBODIES
* Antibodies function in setting of infectious process* Proteolytic enzymes, salt and pH differences
* Antibodies remain stable based on* Sequence of domains
* Single domain consists of* 100 – 110 amino acids folded into compact and stable conformation
* Domains* Variable (V)
* Single V domain in H and L chains
* Constant (C)* Single C domain in L chains
* Three to four (C) domains in H chains
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ANTIGEN BINDING SITES OF IMMUNOGLOBULINS
* Antigen binding sites formed from hypervariable regions* Heavy chain V domain* Light chain V domain
* Hypervariable regions of V domains* Amino acid sequence differences concentrated* Flanked by less variable framework regions* Three hypervariable regions in each V domain* Hypervariable regions also called
* Complementarity-determining regions (CDR)
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ANTIGEN BINDING SITES OF IMMUNOGLOBULINS
* Antigen binding sites vary with size and shape of antigen
* Part of antigen to which antibody binds* Antigenic determinant (Epitope)
* Antigen-Antibody binding based on non-covalent forces* Hydrogen bonds
* Affinity* Strength of binding of one molecule to another by a single binding site
* Avidity* Overall strength of binding between two molecules
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ANTIBODIES AS DIAGNOSTIC AND THERAPEUTICS AGENTS
* Based on specificity and affinity of antibodies
* Both applications require large quantities of identical antibodies* Monoclonal antibodies
* Monoclonal antibodies are produced using hybridoma cell line
* Hybridoma cell line* Derived from single antibody producing cell fused with myeloma
cell (neoplastic plasma cell)
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IMMUNOGLOBULIN DIVERSITY IN B-CELLS BEFORE ENCOUNTER WITH
ANTIGEN
* Immune system capable of producing a limitless number of different immunoglobulins/antibodies
* Mechanism* Genes for IG organized differently
* Genes exist as nonfunctional segments* Variable (V), Joining (J), Diversity (D), Constant (C)
* Genes are inherited in this form* Germline form (germline configuration)
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IMMUNOGLOBULIN DIVERSITY IN B-CELLS BEFORE ENCOUNTER WITH
ANTIGEN
* Expression* Gene segments must be rearranged into functional gene
* Gene Rearrangement* Takes place during development of B-cells
* Mechanism of somatic recombination
* Genes for IG located at 3 chromosomal locations* Heavy chain locus on chromosome 14
* Kappa light chain locus on chromosome 2
* Lambda light chain locus on chromosome 22
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GERMLINE ORGANIZATION OF HUMAN IG HEAVY CHAIN AND LIGHT CHAIN
LOCI
* Lambda light chain locus* Gene segments
* 30 (V), 4 (J) and 4 (C)
* Kappa light chain locus* Gene segments
* 40 (V), 5 (J) and 1 (C)
* Heavy chain locus* Gene segments
* 65 (V), 27 (D), 6 (J) and 9 (C)
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CONSTRUCTION OF LIGHT CHAIN AND HEAVY CHAIN VARIABLE REGIONS
* Light chain* Constructed from 2 segments
* 1 (V) segment
* 1 (J) segment
* Heavy chain* Constructed from 3 segments
* 1 (V) segment
* 1 (D) segment
* 1 (J) segment
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SOMATIC RECOMBINATION
* Performed by enzymes with cut and rejoin DNA
* Directed by * Recombination signal sequences (RSS)
* Recombination signal sequences* Recognition sites for enzymes
* Recombination occurs between different types* 9 / 12 / 7
* 9 / 23 / 7
* Mechanism follows the 12/23 rule* Ensures segments joined in correct order
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MECHANISMS OF GENETIC DIVERSITY IN V-REGION OF IMMUNOGLOBULINS
* Random combination of* V and J segments in light chain genes* V, D and J segments in heavy chain genes
* Addition of P (palindromic) and N (non-templated) nucleotides at junctions of gene segments during recombination* Junctional diversity
* Association of H and L chains in different combinations
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CONSTRUCTION OF B-CELL SURFACE IMMUNOGLOBULINS
* Following rearrangement of VH gene segments, two CH loci are transcribed * IgM
* IgD
* M and D constant segments * Located nearest variable segments
* M and D transcript processed by* Cleavage, polyadenylation and splicing
* IgM and IgD enter endoplasmic reticulum
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SURFACE IMMUNOGLOBULINS ASSOCIATED WITH PROTEINS TO COMPLETE ANTIGEN RECEPTOR
* In ER, IgM and IgD associated with transmembrane proteins* Ig-alpha* Ig-beta
* Transmembrane proteins* Transport M and D to B cell surface* Communication of antigen binding to inside of B cell
* Tails interact with intracellular signaling molecules
* Complex of IgM and IgD with Ig-alpha and Ig-beta forms* B-cell receptor
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DIVERSIFICATION OF ANTIBODIES AFTER B-CELLS ENCOUNTER ANTIGEN
* Mature, naïve B cell has membrane bound IgM and IgD antigen receptors
* Binding of antigen initiates proliferation and differentiation of B-cells into plasma cells
* During differentiation, B cells switch from making immunoglobulin to antibody M and D isotypes
* IgM* Produced in large amounts
* Provides protective immunity
* IgD* Produced in small amounts
* No known function
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MECHANISM OF SWITCHING FROM IMMUNOGLOBULIN TO ANTIBODY
* Surface and secreted forms derived from same heavy chain gene by alternative RNA processing
* Each heavy chain C gene has* Membrane coding (MC) region
* Secretion coding (SC) region
* Mechanism involves a switch in cleavage, polyadenylation and splicing* From pAm region to pAs region
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DIVERSIFICATION OF ANTIBODIES AFTER B-CELLS ENCOUNTER ANTIGEN
* Following antigen activation of B-cells, additional diversification occurs in V domain by* Somatic hypermutation
* Somatic hypermutation* Introduction of random single nucleotide substitutions (point
mutations) throughout V regions of H and L chains
* Mechanism poorly understood
* More common in hypervariable regions (CDRs)
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OUTCOME OF SOMATIC HYPERMUTATION
* Gives rise to some antibodies with higher* Affinity for antigen
* Affinity* Strength of binding of one molecule to another by a single binding
site
* Higher affinity antibodies are produced as immune response proceeds* Affinity maturation
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THE PRIMARY HUMORAL IMMUNE RESPONSE
* Immune response initially produces IgM antibodies then switches to IgG antibodies
* Question* Why switch from IgM to IgG?
* Answer* Limited effector mechanisms for IgM* Range of effector mechanisms for IgG
* Mechanism* Isotype or class switching
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ISOTYPE OR CLASS SWITCHING
* Process by which B cell changes class of IG produced while preserving antigenic specificity
* Involves somatic recombination which attaches different heavy chain constant region to variable region
* Occurs only during active immune response
* Mechanisms involves recombination between* Switch sequences (regions)
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CLASSES, SUBCLASSES AND PHYSICAL PROPERTIES OF IMMUNOGLOBULINS
Subclasses are numbered according to plasma concentration
Classes Subclasses
IgG IgG1, IgG2, IgG3, IgG4
IgA IgA1, IgA2
IgM
IgD
IgE
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FUNCTIONS AND PROPERTIES OF ANTIBODY
* Neutralization* Direct inactivation of pathogen or toxin thereby preventing its
interaction with human cells
* Opsonization* Coating of pathogens for more efficient phagocytosis
* Activation of complement* More efficient phagocytosis
* Direct killing
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IgM ANTIBODY OF THE IMMUNE RESPONSE
* First isotype produced in primary response* May or may not be produced in secondary response
* Produced before B cells undergo somatic hypermutation
* Occurs as pentamer with J chain* Found primarily in blood and lymph
* Multiple binding sites confers high avidity and compensates for low affinity of monomers
* Highly effective in complement activation
* Functions as rheumatoid factor
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IgG ANTIBODY OF THE IMMUNE RESPONSE
* Second isotype produced in primary response
* Primary isotype of * Secondary immune response
* Memory immune response
* Represents approximately 75% of total serum IG
* Four subclassses (1-4)* Different effector functions
* Transported across placenta
* Functions as rheumatoid factor
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IgA ANTIBODY OF THE IMMUNE RESPONSE
* Two subclasses (IgA1 and IgA2) and two forms (monomeric and dimeric)
* Monomeric* Located in blood and extracellular spaces* Predominately IgA1
* Ratio of IgA1 to IgA2 is 10:1
* Functions as rheumatoid factor
* Dimeric* Located in mucous membranes and secretions* Predominately IgA2* Ratio of IgA2 to IgA1 is 3:2 * J chain like IgM
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IgE AND IgD ANTIBODIES OF THE IMMUNE RESPONSE
* IgE* Binds with high affinity to receptors on mast cells, basophils and
activated eosinophils
* Longer half-life when cell bound
* Initiates a strong inflammatory reaction to parasites
* Involved in allergic reactions
* IgD* Antigen receptor on mature B-cells
* No other known function
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